1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC).
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NO.sub.x, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as Pt, Pd and/or Rh for reducing NO.sub.x to N.sub.2 and for oxidizing CO and HC to CO.sub.2 and H.sub.2 O. In this way, the three-way catalytic converter works as a catalyst both for oxidation and reduction.
Various researches have been made to improve the performance of a three-way catalytic converter. One of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO.sub.2) which has an oxygen-storing capacity (OSC); that is, the capacity to occlude gaseous oxygen in the crystalline structure and to release the occluded oxygen from the crystalline structure. More specifically, CeO.sub.2 is added to a three-way catalytic converter for adjusting the oxygen concentration of gaseous atmosphere, so that excess oxygen in the gaseous atmosphere is occluded into the crystalline structure of CeO.sub.2 in an oxygen-rich state for assisting the catalytic converter in reducing NO.sub.x to N.sub.2 while releasing the occluded oxygen into the gaseous atmosphere in a CO- and/or HC-rich state for assisting the catalytic converter in oxidizing CO and HC to CO.sub.2 and H.sub.2 O, respectively.
Meanwhile, there is an increasing demand for shifting the mounting location of the catalytic converter from below the body floor to the exhaust manifold which is near the engine, whereby the catalyst can be quickly warmed up after starting the engine. Due to such a location, however, the catalytic converter may be often exposed to high temperature of no less than 900.degree. C. (or sometimes even higher than 1,000.degree. C. Thus, the catalytic converter needs to provide a high catalytic activity even at such a high temperature. Further, the catalytic converter is also required to provide a high catalytic activity at relatively low temperature before the engine is sufficiently warmed up upon start thereof.
Japanese Patent Publication 5-47263 (which is the granted version of JP-A-63-156545) discloses a catalytic converter for cleaning exhaust gas wherein fine particles of zirconia (ZrO.sub.2) carrying a precious metal (e.g. Pt, Rh) are coated on a heat-resistant honeycomb support together with particles of heat-resistant organic oxide (e.g. alumina) and particles of an oxygen-storing oxide of a rare earth element. While this prior art catalytic converter aims to provide a high catalytic activity at a high temperature, it does not pay any attention to catalytic activity at relatively low temperature.